Note: Descriptions are shown in the official language in which they were submitted.
1 170379
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A CIRCUIT FOR TRANSFERRING SIGNALS
BACKGROUND OF THE INVENTION
This invention relates to a digital conferencing
system and more particularly, to a time slot interchanger
for buffering information from one time slot bus to a
second time slot bus.
It is desired to construct a time multiplexed
system with time slot interchange capability and with the
ability to conference together a group of time slots for a
given communication connection. A typical prior time slot
interchanger is constructed using a time slot assignment
(TSA) memory and two sample buffer memories. The TSA
memory contains, for each time slot, an instruction which
points to a location within the sample buffer. The system
processor loads the instructions into the TSA memory for
each communication connection. A time slot clock operates
to sequence through the TSA memory and for a given time
slot address in the TSA memory a sample buffer memory
address is provided. The data sample associated with the
given time slot is loaded into the sample buffer at the
location therein identified by the address obtained from
the TSA memory. This prior disclosure, while advantageous
- from the time slot interchange standpoint, requires the
time slot address memory to have as many memory locations
as there are input bus time slots. In addition, when
establishing the sample buffer address in the time slot
address memory, only one sample buffer address can be used
for each time slot address memory. Thus, this prior
system is not designed to form conferences between time
slots, since for conference capability, the same time slot
sample must be stored in more than one buffer location.
;~ 30 SUMMARY OF THE INVENTION
In accordance with an aspect of the invention
there is provided a circuit for transferring signals
between an input and an output, said input and output
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having established thereat a plucality of time shared
positions, each position having a unique identity, said
circuit characterized in that there is included a first
memory having address locations therein each location
corresponding to a particular one of said output time
positionsj each location adapted to store thereat the
identity of an input time position, means for establishing
address inputs for said first memory, said inputs
consisting of the identities of said input time positions,
and means controlled by an association between an
established address input and a stored time position
identity at a particular location with said first memory
for transferring a signal from the input time shared
position associated with said matched identity to the
output time shared position associated with said
particular first memory storage location.
A time slot interchanger is disclosed for
selecting and buffering digital signals between buses.
The arrangement is particularly useful in forming
conferences
1 170379
in a distributed digital time division system. In one
embodiment, a clock is used to generate time signals
corresponding to the first bus time slots. These signals
are sequentially provided to an associative memory, and
when a match occurs between the provided signal and a
priorly stored time slot identity, an enable signal is
provided. Each associative 'ocation has comparison logic
and an enable output sisnal. Each enable signal has a time
identity with a priorly established first bus time slot and
a physical identity with a particular second bus time slot.
The enable signal causes a write of the time slot signal
corresponding to the time identity into a storage location
in a buffer memory corresponding to the particular time
slot of the second bus. The signal samples are then
sequentially removed from the buffer memory.
In this embodiment, the associative memory, also
known as a content addressable memory (CAM), and the sample
buffer need only contain as many locations as there are
time slots on the second or conference bus. Thus, for a
256 time slot main bus and a 32 time slot secondary bus
there need only be 32 locations for both the CAM and the
sample buffer. For a given time slot transfer the central
processor loads into the CAM, at the storage location
therein associated with the desired output bus time slot,
the identity of the main bus time slot. ~hen a match
occurs between a time slot clock and a time slot address
loaded into the CAM, an output signal is provided to the
sample buffer at tlle corresponding location, thereby
allowing the given time slot sample to be written into that
location in the buffer. The sample buffer is then
sequentially read onto the output bus.
Since the same time slot identity can be written
into multiple CAM locations, the same time slot sample can
be loaded into multiple sample buffer memory locations. In
this manner multiple conferences can be obtained on the
output bus.
1 170379
Brief Description of the Drawing
These and other objects and features, together
with the operation and utilization of the present
invention, will be more fully apparent from the
illustrative embodiment shown in conjunction with the
drawing in which
FIG. 1 is a broad block diagram showing a
distributed communication system having the time slot
intercnange alyorithm of our invention shown within the
Network Processing Element (NP~) of a system port,
FIG. 2 is a block diagram showing the arrangement
of a system port,
FIG. 3 is a block diagram of the network
processing element of each system port,
FIG. 4 is a schematic of the Associative
Conference Buffer,
FIG. 5 is a schematic of the Bus Select Register,
FIGS. 6, 7, 8, and 9 show the Content Addressable
Memory, the Gain Value ~uffer, and the Sample Buffer in
detail,
FIG. 10 shows how FIGS. 8 and 9 should be
arranged,
FIG. 11 illustrates the operation of the time
slot interchanqe function between an input bus and an
output bus,
FIG. 12 shows a timing table for controlling a
dual access memory.
General Description
The time slot interchange function, as performed
~0 in a digital conferencing system, is shown in FIG. 1. The
operation of the overall system will be detailed
hereinafter. However, prior to entering into such a
description it may be beneficial to concentrate a moment on
the interchange of time slot signal samples using content
addressable memory 600 and sample buffer 800, both shown in
FIG. 1 and shown in expanded form in FIG. 11.
11703 ~
From the chart it will be seen that for at least
some portion of time it is desired to interchange signal
sample A, which has an input time slot identity of time
slot 2 onto the output bus at time slot 30. Similarly, it
is desired to interchange signal sample B, having an input
bus identity of 5 onto output bus time slots 1 and 31. The
same type of interchange is desired for input bus time
slots 7 and 254.
In this example, the system microprocessor, via
bus 401, writes the coded identity for time slot 254 into
position 0 of CAM 500. Time slot identity 5 is written
into CAM 600, position 1; time slot identity 7 is written
into position 2; time slot identity 7 is written into
position 29; time slot identity 2 is written into position
30; and time slot identity 5 is written into position 31.
As will be seen, each of the physical positions
of CAM 600 corresponds to a particular one of the output
bus time slots and also corresponds to a particular one of
the storage locations within sample buffer 800. Once the
time slot identities are written into CAM 600, they remain
until changed under control of the system processor. Any
set of interchanges can be updated without the need to
change the entire memory, if that is so desired.
A time slot counter provides a sequence of time
slot identity input signals over bus 506. CAM 600 is
arranged, in the well known manner, and as will be
described hereinafter, to compare the input signals against
information stored at the various storage locations
therein. Thus, when bus 606 provides the identity 2, the
memory element in physical position 30 provides a match
signal via cable 605 to sample buffer 800. The clock
signals on bus 606 are synchronized with the time slots on
input bus 809, 810. Thus, the match signal to position 30
from CAM 600 opens that position in sample buffer 800 for
the storage of the signal sample currently on the input
bus, in this case signal sample A. Accordingly, signal
sample A is stored in buffer 800 at position 30. When the
1 ~70379
clock signal goes to 5, positions 1 and 31 of buffer 800
open and signal sample B is stored at both locations
therein.
At the completion of the entire frame, i.e., when
the time slot counter reaches the count of 255, sample
buffer 300 is examined sequentially from 0 through 31 and
the information stored therein is read out onto the output
bus 811. The signal samples are then available on the
output bus during the time slots desired.
Detailed Description
FIG. 1 illustrates a communications system in
which the conferencing control is distributed among the
system ports 200-1 to 200-N. The time slot interchanger
operates within this system, but will also operate with
other switching or transmission systems as well. Each such
port of the system serves a number of terminals, such as
station Sl. Serving the system ports there is shown a dual
bus digital system, having Bus A and Bus B, with common
system control 100. The system control has bus interface
and timer 101, call processor 103 and tone source/signal
detector 102. The call processor operates to take in
stimulus from the stations via the ports and to control
station interconnection by establishing the time slots
which are to be used for each station. Processor 103
provides control information to the system ports indicative
of the identity of the time slots which must be combined
for a given conference. This operation is well-known. The
control section also includes tone signal generator and
detector 102 for generating and detecting call progress
tones. The system shown handles voice signals, as well as
data, between the various stations. The conference summing
feature would be used for voice conferencing in this
system.
The system port shown in FIG. 1 has been expanded
in FIG. 2 to show the circuit elements. I/O buffers (2n4,
205~ interface the port circuits to the high power-high
fanout system buses A and B. The Network Processing
1 i703~9
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Elements 3no (~PE), of which only three are shown, process
and control the signals between the stations and the
buffered buses 321, 322. The NPEs transmit signals from
each of the stations onto either of the two buses and
receive signals for each station from either bus. The NPEs
operate to perform the distributed conferencing function in
the manner to be discussed hereinafter.
Each network processing element as shown is
capable of handling data to or from four stations. Station
interface circuits 201 contain either codecs or digital
station formatting circuits to send or receive samples from
a station. Each station interface circuit operates to
properly format the samples coming to and from a digital
station, and operates to convert between analog and digital
transmission for an analog station.
Line 106 handles bidirectional communication with
station Sl (FIG. 1), while line 107 is associated with
station S15 (FIG. 1). This configuration has been chosen
for manufacturing convenience and any number of stations
may be associated with a station interface and any number
of interfaces can be associated with an NPE and any number
of NPEs may be associated with a system port.
In FIG. 2 there is shown microprocessor
controller 202 and control channel interface 203.
Microprocessor controller 202 assigns transmit and receive
time slots to each of the MPEs over bus 4nl. Control
channel interface 203 allows microprocessor 202 to
communicate over either bus 321 or bus 322 via bus A or bus
B to Call Processor 103 via Bus Interface 101 (FIG. 1).
There are two buses designed into the illustrated
system to double the capacity of the system. Each bus runs
at 2.048 MHz sample rate allowing 256 time slots per bus.
Having two buses allows up to 512 time slots but the use of
two buses is not required for the distributed conferencing
or time slot interchange inventions.
I/O buffers 204 and 205 operate in either
direction and are under control of the NPEs or control
1 170379
channel interface 203. Each of the buffers normally
receives sa~ples from the bus during all time slots, but,
when any particular NPE requires a transmission on a
particular time slot, that NPE will force the buffer to
transmit while simultaneously outputting its data to the
corresponding bus (321 or 322). The NPE will signal the
buffer via the TEA (or TEB) line causing the corresponding
buffer to transmit the data on bus 321 (322) onto the
system bus A (B).
A call is established in the system by call
processor control 103, FIG. 1, as a result of a stimulus
from a station over a line, such as line 106. This
stimulus is received by microprocessor controller 202
(FIG. 2) which sends a stimulus signal through control
channel interface 203 over either bus A or bus B to call
processor 103, FIG. 1. The call processor establishes
which time slots are to be used for the call and sends a
response signal back over either bus A or bus B to control
channel interface 203 of the ports involved. The
microprocessor controller at those ports then programs the
NPEs to transmit and receive on specified time slots for
the duration of the call.
TIME SLOT CONTPOL
NPE 300 described in FIG. 2 is expanded in FIG. 3
to illustrate its operation in the system. For purposes of
illustration let us assume the NPE shown in FIG. 3 is
associated with four stations, namely A, B, C and D.
Transmission from Station A is via line 301-1 while
transmission to station A is via station 301-2. It should
be kept in mind that transmission from any station A
through D may terminate at any other station A through D
served by the same NPE or by any other NPE. The conference
here is restricted to one NPE for the purpose of
illustration. Transmission multiplexers 311 and 312
transmit samples from each station onto the buses during
time slots established by the Associative Conference Buffer
400 (ACB). Simultaneous to the samples being transmitted
1 17037~
onto the bus, samples are being received from the bus and
sent to each of the four stations A-D, all through AC~ 400
and conference circuit 331. ACB 400 is programmed by
microprocessor controller 202 (FIG. 2) over bus 401 ~o
receive data samples from specific time slots and to group
the data samples from these time slots for conference
summing and subsequent transmission to the proper stations.
The sums are presented to the proper station via
synchronizers 301-2, 302-2, 303-2 and 304-2. The
conference sums are developed in a time multiplex fashion
independently for each of the four stations. The ACB
reorders the samples in the manner to be more fully
discussed, so that the conference logic will produce four
independent sums each of which will go to the corresponding
station interface. Conference circuit 331 receives 32
independent samples from the ACB. The 32 samples are
conferenced in four groups of eight samples each. The
first eight samples of the 32 are added together and sent
to station A over synchronizer 301-2. The second eight are
added together and sent over synchronizer 302-2 and so on
for the third group of eight and the fourth group of eight.
If the station is not receiving data at the time, all of
its samples would be zero. This result could be
accomplished either by all the signals being zero or all of
their corresponding gains being zero. Under the present
invention, the yain of each signal for each station is
separately controllable.
ACB 400 acts to remove data from specified time
slots of either of the buses and combine that data with
specified buffering information (gain) for each time slot
so that the conference can be controlled with respect to
gain for any conferee. The value of such conference
control lies in the fact that for different combinations of
stations different gain values can be appropriately
selected so that conferencing can be performed without vast
differences in volume from different stations.
O 3 ~.' 9
The ACB consists of four separate pieces of
memory; namely a Content Addressable Memory 600 (CAM), a
Sample Buffer 800 (SB), a Gain Value Buffer 700 (GVB), and
a Bus Select Register (BSR) . The CAM and the GVB are
programmed over bus 401 by a microprocessor controller.
The CAM is programmed to select time slots from the bus.
The data in those time slots is loaded into the SB in the
programmed order. The GVB is loaded by the microprocessor
and each gain is used with a corresponding sample in the
10 SB . Time Slot Counter 310 establishes both when the CAM
will react to programmed time slots and when the samples
along with their corresponding gains will be read out of
the SB and GVB. As discussed, the read out will occur in a
sequential order and will consist of 32 samples which will
be in four groups of eight samples each.
The BSR 500 is also programmed over bus 401 and
operates to select which bus each of the samples loaded
into the SB come from. This bus selection is handled by
bus select control 801.
Summarizing briefly, the NPEs (FIG. 2) in each of
the system ports operate to control the movement of data on
and off the bus. For this to occur in the proper sequence,
the local Time Slot Counters 310 for each NPE must be
synchronized over the entire system. This is accomplished
via bus control 100 over buses A and B by bus interface and
timer 101 in bus control 100 shown in FTG. 1. Bus
interface and timer 101 contains a timing circuit which
generates a clock and a frame signal. The clock signal is
a 2.048 M~z signal which is the speed of the buses and the
frame is a 8 k~z synchronization signal. The clock and
frame signals go to every system port and are buffered at
each port and provided to each NPE to count and reset the
local time slot co~lnter. This insures that, even though
system control is distributed, all of the NPEs realize
3S equivalent Time Slot Addresses.
~70379
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TIME SLOT INTERCHANGER - DETAILED DESCRIPTION
Associative Conference Buffer 400, shown in
expan~ed form in FIG. 4, operates to accept gain values and
time slot addresses from the microprocessor controller over
bus 401. Time slot addresses determine which time slots
will be written to and read from the bus. The gain values
are loaded into GVB 700 via I/O register 704. The time
slot addresses are loaded into CAM 600 via I/0 register
603. When a call between a given group of stations is
established in the system the microprocessor at each port
loads the gains values and the time slot addresses of the
call for the duration of the call.
~ ith the gain values and the time slot addresses
loaded, the CAM 600 controls SB 800 to load samples from
either bus 809 or bus 810. The samples are only loaded
into SB 800 when a corresponding CAM location holds the
time slot address of that sample. The precise manner in
which this is accomplished is detailed hereinafter. The
samples are held in the SB until being sequentially read
out over bus 811 to expander 309 (FIG. 3).
CAM 600 recognizes the time slots on buses 809
and 810 by comparing the time slot address bits zero to
seven (TSA0 to TSA7) over line 606 with the CAM's stored
time slot addresses. Each CAM location individually
compares its 8 bit data with the 8 bit data on line 6n6.
If these are equal, that CAM location produces a match
signal over the corresponding line of 605. This match
signal causes a write into the corresponding SB location
from one of the input registers (807 or 808). The CAM
therefore can recognize 256 (0-255) distînct time slots or
time intervals on bus 809 or 810. Each of these 256 time
intervals could produce a write signal to the S~ for
writing the sample on the bus during that time slot. The
scan or read out process of SB 800 is controlled by time
slot address 3 through 7 (leads TSA3 to TSA7) via scan
selector 701. The samples are thus scanned out of the SB
over bus 811 at a rate one-eighth the rate that samples are
1~7037'3
presented to the SB. This results since the CAM recognizes
time slot address bits n to 7 which are changing eight
times faster than the time slot address bits 3 through 7.
Samples are also written into the SB at this 1/8 rate but
not uniformly, because this write may occur for any of the
256 time slots. Bus 811 therefore has 32 time slots
whereas bus 809 and bus 810 each have 256 time slots.
SE 8~0 and CAM 600 along with time slot counter
310 perform a time slot interchange function which
selectively removes samples from desired time slots on bus
809 or bus 810 and presents these samples in a specified
order to bus 811.
The reordering process is graphically ~shown in
FIG. 11 where samples are taken from an input bus (either
bus 809 or bus 810) and transmitted to an output bus (bus
811). For purposes of illustration let us assume that on
the input bus we have samples A, B, C and D which represent
samples from four stations served by an NPE, such as shown
in FIG. 3. It is, of course, understood that the samples
can come from any station in the system and not only from
the stations associated with this particular NPE. The main
system processor has established the order shown in FIG. 11
where time slot address 2 has a sample from station A while
time slot address 5 has a sample from station B and so on.
Let us further assume that we have a four party conference
between stations A, B, C and D. With respect to the output
bus we will concern ourselves with the samples for stations
A and D recognizing of course that there would be similarly
buffered samples for stations B and C which have not been
shown. It will be recalled that the 32 samples on the
output bus are conferenced in four groups of 8, the first
group for station A and so forth. Thus, samples D, B and C
go to station A while samples C, A, and B go to station D.
Each group of samples will be added together and sent to
the corresponding station over synchronizers 301-2 to 304-2
of FIG. 3.
1 17~37~
The interchange between the input bus and the
output bus is controlled by CAM 600 such that CAM 600 is
preloaded from the system controllers described priorly, to
contain a 254 in position 0, a 5 in position 1, a 7 in
position 2, a 7 in position 29, a 2 in position 30 and a 5
in position 31. For the duration of this call, these
numbers will remain in the physical position shown. Thus,
the central processor need only communicate with this NPE
once per call unless a new station is added or subtracted
from the conference call.
The operation then is to take the sample from
input time slot 254 (sample D) and move it to output bus
time slot 0. This occurs because CAM 600 operates, as will
be more fully detailed, to compare each time slot identity
against a stored number and to provide an output signal
when a match occurs. Thus when the TSC reaches 254, a
signal is provided from location 0 of CAM 600 to location 0
of SB 800. This signal serves to allow the data currently
on the input bus to become stored at location 0 of SB 800.
The second location, location 1 of CAM 600, has been loaded
with a 5 indicating that the sample to be loaded into
location 1 of SB 800 will come from time slot 5. This
first and second SB location will then constitute the first
and second time slots of the output bus. Similarly, CAM
locations 2, 29, 30 and 31 are programmed with the time
slot address of the input bus and their physical location
in the CAM determines what time slot the samples will
occupy on the output bus. As time slot counter 310 cycles
from 0 through 255, its output is provided over bus 606 to
CAM 600. Each time there is a match between the time slot
count and a number stored in the CAM the physical location
of the match in the CAM causes a write pulse to be present
at the same physical location of SB 800. Thus the sample
corresponding to that time slot from the input bus is
loaded into the SB at that location.
Thus, as discussed above, when time slot address
2 appears on bus 606 memory location 30 of CAM 600 provides
1 170379
a write pulse into memory location 30 of SB 800 thereby
causing the sample associated with time slot address 2
(which is sample A) to be written into location 30 of
sample buffer 800. ~hen the time slot address reaches 5,
locations 1 and 31 of CAM 600 provide write pulses to
locations 1 and 31 of SB 800 thereby causing sample B to be
written into those two locations concurrently. At the end
of a frame, the SB will be filled and a sequential read
will begin so as to read the stored data to the output bus
in the proper sequence and during the proper output time
slot. In this manner, there is generated a 32 time slot
bus (811) to present samples to the conference circuit.
Returning to FIG. 3 the time slot interchange is
being controlled by CAM 600 and SB 800. The interchanged
output samples are presented to expander 309. In addition,
the Gain value Buffer 700 (GVB) provides a gain value for
each buffered sample. Scan address word selector 701
controls both the SB and the GVB so that each of the buffer
locations has a corresponding location in the other buffer.
Thus each of the 32 samples read onto bus 811 from the SB
has a corresponding prestored gain sample read onto bus
707. The gain sample is then supplied to multiplier 308 of
conference circuit 331 (FIG. 3). Each sample coming onto
bus 811 passes through ylaw expander 309 and then is
multiplied with its corresponding gain sample on bus 707.
This establishes the gain coefficient for each of the
samples on an individual basis. With this approach, the
gain of each sample for each station may be tailored to
that station, and may be further tailored depending upon
the origin of the sample.
In groups of eight, these samples are then
accumulated by accumulator 307 and the accumulated sum is
then recompressed by ~law compressor 305 and then sent out
to the proper output station via one of the output
synchronizers (301-2 to 304-2).
1 1703~9
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ASSOCIATIVE CO~FERENCE BUFFER MEMORY ARR~NGEM~NT
Four basic memory systems are included in the
associative conference buffer 400, namely Bus Select
Register 500 (BSR), Content Addressable ~emory 500 (CAM),
Gain Value Buffer 700 (GVB), and Sample Buffer 800 (SB).
F`IGS. 5, 6, 7 and 8 provide the details of the operation of
each of these memory systems. BSR 500-shown expanded in
FIG. 5, consists of straightforward readable/writeable data
flip flops. Decoder 501 selects one of four groups of
eight bits to be written eight bits at a time from data b~s
401. The outputs of these four, eight bit registers are
used to determine bus selection for samples written into SB
800. The BSR functions to decide which bus, either bus 80g
or 810, is to provide the samples to be written into the
~B. This is accomplished on an individual basis for each
of the SB locations. Without a dual bus structure, the BSR
would not be required.
CAM 600 is expanded in FIG. 6 showing the
structure of each of the bit cells (e.g., 604) in the
memory array and the manner in which the address decoder
connects to this memory array. The CAM is read and written
as any other standard memory through I/O register 603. The
address is decoded by address decoder 502 to select 1 of
the 32 (0-31) eight bit locations. When one of these
locations is selected, the data to be written is taken via
the I/O register and presented on the data line (D0- ~ and
D0-D7) to the selected memory bit cells such as bit cells
~-~ to 0-7. Each bit cell 604 is a static memory cell
consisting of resistors 6R1l 6R2, and transistors 6042 and
6045 which form the latching portion of the cell. The cell
is accessed for either the read operation or the write
operation through transmission gates 6041 and 5048. The
transmission gates are turned on or off by the address
select line coming from address decoder 602. For data to
be written into cell 0-0, the I/O register presents the
data on lines DO and DO and then the address decoder line 0
turns on transmission gates 6041 and 6048 allowing the data
1 1703 ~9
on lines DO and DO to set or reset memory cell 0-0. The
read operation is performed in similar manner. The address
decoder, line 0, turns on transmission gates 6041 and 60~
and tle data stored in bit cell 0-0 then propagates out on
lines DO and DO to the output register portion of I/O
register 603.
In addition to the above-described standard
memory operation there is associative recognition circuitry
in each bit cell. For bit cell 0-0 this circuitry includes
transistors 6043, 6044, 6046 and 6047 which perform an
EXCLUSIVE OR operation between the data bit that is stored
in cell 0-0 and the data bit that is brought in on line
TSAO and TSAO. This EXCLUSIVE OR along with the EXCLUSIVE
ORs in bits 0-1 to 0-7 compare the data from time slot
counter 310 (TSA0 and TSA7) with the data stored in CAM
location 0, and when they match, line 620 goes high. Line
620 (bit line 0) only goes high when each bit of CAM
location 0 equals each bit of TSA0 to TSA7. The eight bits
as a group hold a prestored time slot address and all are
compared simultaneously with the incoming time slot
address. When all eight stored bits match all of the bits
on line 606, line 620 becomes active indicating this match.
Thus a match signal is produced from lead 0 of 605. Each
of the 32 eight bit locations in the CAM have identical
comparison circuits and operate to independently compare
the data stored therein with the data on line 606.
Turning again to FIG. 11, as discussed
previously, location 0 can be stored with a binary 254 as
an eight bit number. Thus, there are 32 independent match
lines, each of which will indicate when the data stored in
the corresponding location of the CAM equals the data on
line 606.
DUAL ACCESS MEMORY STRUCTURE
The gain value buffer is shown expanded in FIG. 7
and consists of an NMOS memory array of prior art modified
to provide dual access capability. Thus, memory 700 can be
accessed either via register 703 or via register 704, each
1 170379
- 16 -
operating with two independent addresses and with two
independent data buses.
Bus 401 can access for read or write purposes any
of the 32 locations selected by address decoder 705.
Simultaneously, and independently, bus 707 can read out
any of the 32 locations selected by address decoder 701.
Both buses are extended as bit line pairs through all of
the memory locations and access on either bus does not
restrict access on the other bus. Bit line pairs are used
as set/reset lines for write operations and as differential
outputs for read operations. Bit line pair 0 and 0 of
register 704 is extended to bit cells 702 of the top row
(0-0 to 31-0) and bit line pair 0 and 0 of register 703 is
also extended to these same cells. Access from bus 401 is
controlled by the microprocessor. This microprocessor
writes gain values into the locations to be available with
corresponding samples that will be processed by the SB-CAM
memory arrangement.
In a non-modified NMOS memory array, one set of
bit line pairs and one I/O register with one address
decoder would be connected to the memory array. For
purposes of this discussion we will assume this to be
decoder 701 and register 703. Any read or write operation
is a two step process. The first step is to precharge all
the bit line pairs. Thus lines 0 to 5 and 0 to 5 are
driven to a high state by circuits in the register 703.
This prevents the lines from changing the data in the bit
cells during the next step. For a read, the next step is
to turn off the precharge drive and turn on one of the
word select lines from the decoder 701. Turning ~ff the
precharge drive leaves the bit lines capacitively charged
high, while the word select line turns on corresponding
transmission gates 7021 and 7025. These transmission
gates allow the bit cell to pull down one or the bit lines
(0 or ~ depending on the stored data). The bit line pair
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- 17 -
thus conforms to the data stored in the selected bit cell
and the register then latches this data for output. The
bit cell resistors 7Rl and 7R2 are high value to minimize
memory power consumption whereas the transistors 7023 and
7024 are high power to be capable of pulling either bit
line low. The precharge is required because the resistors
are not capable of pulling the bit lines high.
For a write, the next step is to replace the precharge
drive with the drive of the input data, and turn on one of
the select lines. The input data overrides the precharge
and the bit cell data causing the cell data to be set or
reset depending on the input data. Thus the selected cell
is written into. The dual bus scheme allows a two phase
operating memory system whereby two independent sets of
I/O registers and word selectors can access all of the
same memory cells on opposite phases of a clock. Thus, as
shown in Fig. 12, it can be shown that when one of the
registers, such as input/output register 703 is in the
precharge mode the actual flip-flop of all memory cells
are isolated from that register's bit lines and during
that time the other register, such as output register 704,
can be in the read/write phase actually accessing any
cell. This alternate operation is controlled by
oppositely phased clock pulses as shown in FIG. 12. It
prevents the potentially disasterous condition of having
both bit lines simultaneously selected to the same bit
cell. With the GVB, the bus 707 is only used for read
operation.
The dual bit line/dual phase operation allows the gain
buffer to effectively double its speed so that twice as
many accesses can occur through independent ports in the
same time interval.
This same dual phase arrangement is utilized with SB
800, as shown expanded in FIGS. 8 and 9. The sample
buffer is further expanded by having three bit line pairs
1 170~79
and three access ports and bus selection logic for two of
the three access ports. Address selection lo~ic for the
output port (bus 801) is shared with the GVB. The other
two ports (A and B) come from bus ~10 and 809 through input
registers 807 and 808. Address and port selection for A
and B is provided by the CAM and bus selection logic 801.
Samples are simultaneously present on buses A and B coming
from input registers A and B. The bus selector of each SB
location controls from which bus data will be written into
its SB location. This arrangement provides a flexible
three port memory system in which two ports are inputs and
may perform simultaneous writes to more than one location
and from either of two buses, while the third port is an
output and provides simultaneous reads from a third bus to
be scanned out for the conference circuit. Since the two
buses A and B are both running on the same phase this could
create a conflict for writes except that the bus selection
logic ensures that for any given location only one bus will
provide the write data. The third bus, bus 811, is running
on the opposite phase and thus cannot conflict with either
bus A or B.
The bus selectors take signals from both the CAM
600 and the BSR 500. The CAM determines when a sample on
bus A or B is to be loaded into the corresponding SB
location. Its write pulse is directed to write from either
bus A or B on the basis of the corresponding BSR bit. As
shown in location 0-0 bit cell 805, transmission gates 8053
and 8058 allow data from bus B to be written into the bit
cell whereas transmission gates 8052 and 8057 allow data
from bus A to be written into the bit cell. Only one of
these two sets of transmission gates is enabled at any
given time as determined by the corresponding bus selector.
Conclusion
While the invention has been illustrated in
conjunction with a time slot interchanger conferencing
system, such an application is only one embodiment, and it
would be obvious to one skilled in the art to use the
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invention to move data samples from one input to another
input, whether or not those inputs are associated with
stations, lines, trunks or auxiliary circuits, or from a
transmission line to a Memory array for later delivery.
The memory storage array could be arranged to have a number
of storage levels, each level corresponding to one full
cycle of the input signal. Thus, it would be possible to
store several frames of the input signal in the memory for
later delivery. Possibly, such an arran~ement could find
application in packet switched systems where buffering is
required.
It should also be obvious to combine the various
memories into a single memory structure, possibly also
incorporating the input and output buffers and buses. The
clock signal can be internally generated, and separate
clock signals may be used for gating purposes.
